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Plastics

 

Introduction

The word plastic comes from the Greek word ‘plastikos’, which means mouldable. Plastic is one of the few new chemical materials, which pose environmental problem. Polyethylene, polyvinyl chloride, polystyrene is largely used in the manufacture of plastics. According to an estimate more than 100 million tonnes of plastic is produced every year all over the world. In India it is only 2 million tonnes. In India use of plastic is 2 kg per person per year while in European countries it is 60 kg per person per year while that in US it is 80 kg per person per year. Plastic in the environment is regarded to be more an aesthetic nuisance than a hazard, since the material is biologically quite inert. 20% of solid municipal wastes in US is plastic. Non-degradable plastics accumulate at the rate of 25 million tonnes per year. These materials have molecular weight ranging from several thousands to 1,50,000. Excessive molecular size seems to be mainly responsible for the resistance of these chemicals to biodegradation and their persistence in soil environment for a long time.

Synthetic polymers are easily molded into complex shapes, have high chemical resistance, and are more or less elastic. Some can be formed into fibers or thin transparent films. These properties have made them popular in many durable or disposable goods and for packaging materials. Plastics can be divided into two main groups; thermoplastics and thermosets (stiff and not elastic).

Most commonly used polymers and their uses:  

Sr. No. Polymer Resin Common Use
1 High Density Polyethylene (HDPE) Rigid containers
2 Low Density Polyethylene (LDPE) Package films, bags
3 Polypropylene (PP) Wrappers, linings, boxes, crates
4 Polystyrene (PS) Foams, insulations
5 Polyvinyl chloride (PVC) Rigid containers, films
6 Polyethylene terphthalate (PET) Soft drink containers
7 Polycarbonate (PC) baby bottles, sports water bottles

Why Plastic is Popular?

  1. Cheap
  2. Durable
  3. Disposable
  4. Convenient for Packaging
  5. Can be molded into Complex Shapes
  6. Inert
  7. Most Energy Efficient Option
  8. Creates Least Air Emissions during Manufacturing
  9. Produces Least Solid Waste by Both Weight and Volume (at equal container recycling rates)
  10. Packaging without plastics results in:
    1. More packaging waste by weight — more than 4 times greater
    2. More packaging waste by volume — more than 2 1/2 times greater
    3. More energy used in manufacturing and distribution — more than 2 times greater
    4. Higher cost of packaging — more than 2 times greater.

According to several studies compiled by the Association of Plastics Manufacturers in Europe, the use of plastics yields large energy savings.

To illustrate, a truckload of bottled water in glass bottles is comprised of 57 percent water and 43 percent glass by weight; while in plastic, the load is 93 percent water and 7 percent plastic. The use of plastic results in an approximately 40 percent reduction in overall motor fuel consumption and the associated exhaust emissions.

Properties of Plastics

  1. Gloss (EPG’s Depart (PVA based)
  2. Transparency (Lacca (PLA based) of Mitsui Chemical)
  3. Density – Usefulness in sedimentation in aquatic environment (PP – Low, PHB – High)
  4. Transparency to UV – can cause chemical oxidation – PE – Transparent, Soya-based – blocks UV)
  5. Gas barrier properties – Important in food packaging (Good in gluten-based and PV-based plastics)
  6. Oxygen permeability of edible polymer films
  7. Water vapor transmittance – (Resistance expected) Important in food packaging. Minimum in PP and PS
  8. Modulus  Resistance to deformation (Low for starch, PHA) Blend with polymers fillers
  9. Glass transition temperature (Temperature at which a polymer changes from hard and brittle to soft and pliable form. (Important for processing) Mazin (PLA based High Tg)
  10. Melting temperature – Important for procesability
  11. Hydrophilicity or Water resistance – Important for disposable diapers and sanitary napkins (PLA water resistant, Depart water soluble)
  12. Flexural Strength – Needed to break the sample  (Good in PP, PS, hard PLA, nylon, Less in PE)
  13. Hardness – Procesability affected
  14. Antistatic properties – Suitable for electronic packaging

Groups of Plastic

Conventional plastics provide a broader range of material properties than Poly-lactic acid (PLA) and Poly-hydroxy-alkonates (PHA), but they are not biodegradable. Plastics can be divided into two main groups; thermoplastics and thermosets.

Thermoplastic: A material that can be molded and shaped when it’s heated. Linear polymers that are not cross-linked and that are not strongly hydrogen-bonded to adjacent polymer chains generally possess this characteristic. Thermoplastic properties are needed to be able to form a material by extrusion. Thermoplastics can be melted and reshaped many times over when heated, and hence can be recycled.

Thermoset: A hard and stiff material. Thermosets are different from thermoplastics, Thermosets are crosslinked so they are not moldable. Also they are different from crosslinked elastomers. Thermosets are stiff and don’t stretch the way elastomers do. Thermosets are like concrete, because they can only be shaped once. They are usually rigid and will not flow again even when heated, making recycling very difficult.

Structures of commonly used Plastics:

plastic

Market for Plastics in India

  • Per Capita consumption of plastic in India – 2kg/person/year
  • Per Capita consumption of plastic in developed countries – 60 kg/person/year
  • Per Capita consumption of plastic in US – 80-90kg/person/year
  • Per Capita consumption of plastic of world – 15kg/person/year
  • The projected demand for plastic in India is about 25 lakh tones/year by 2001.
  • 20,000 processing units by 2000
  • Domestic demand for plastic is expected to cross 4 million tones by the year 2001-2.
  • Plastic 1-4% in solid wastes in India while in USA it is 30% of solid wastes

India is largest manufacturer of thin carry-bags and exports fetched Rs.32 crores. 8-10 micron thickness plastic bags are made out of recycled plastics. With repeated recycling- strength reduces, appearance becomes repulsive and there is no resale value.

1 kg of thin plastic bags = 1000 pieces, Milk bags – 25 micron thickness

Provision to ban plastic bags is in Section 5 of 1986 EPA.

Ban on plastic bags of < 20 micron thickness

Fine for individual violators – Rs.2000/- and vendors – Rs.1,50,000/-

Approximate Breakdown of the Global Plastic Production

Polyethylene terephthalate 6%

Polyethylene 40%

Polyvinyl chloride 20%

Polypropylene 19%

Polystyrene 9%

= Approximate Total 94%

The most commonly used types of plastics are PO, PP, PS, PVC, PET, PC, PU,  polyacrylates, polyvinyl acetates, and polyamides. These synthetic polymers are typically made from the naphtha fraction of petroleum or natural gas; and are heavy pollutants as they are not biodegradable. We are living in a “throw away” society and as a result, millions of tons of plastics end up in landfills, the ocean, and the shores. Even if this practice were to stop today, plastic waste would continue to wash upon our shores for hundreds of years. This has significantly eroded the marine life, as millions of marine animals die each year; and there is clear evidence that this trend will escalate because the global thirst for these materials is on the rise.

The use of plastics in our everyday life is nearly boundless. Due to its low cost of production and versatility, no alternate emerging product is likely to replace the nearly ubiquitous presence of plastics. The current global production level is about 250 million tons and its growth will continue to be robust globally. Plastics are preferred as they are light, durable, resist deterioration, and the markets they cater to are extensive: food, textiles, furniture, electronics, vehicle parts, photography/videography, coatings, construction, enclosures, bottles/containers, and many more.

The common reason these plastics are produced in such abundance is price. These plastics can be used in many applications, often with superior properties, for a lower cost then other materials or plastics that could be used in the same application.

Various Fields of Applications for Plastic

Fields of Applications

Degradability

Depending upon their properties polymers can be referred to as –

  1. Biodegradable
  2. Compostable
  3. Hydro-biodegradable
  4. Photo-biodegradable
  5. Bioerodable
  1. Biodegradable plastics are degradable plastic in which the degradation results from action of naturally-occurring (i.e. no human interference) microorganisms like bacteria, fungi and algae (ASTM D6400-99) Biodegradability does not necessarily mean compostability.  Biodegradability is one of the components of compostability.  Compostability in addition, requires disintegration of 90% of the material to a size less than 2mm, and also mandates that the compostable material does not create any eco-toxicity in the soil. By composting biodegradable plastic along with the other biodegradable waste, we can generate much-needed carbon-rich soil (humus) instead of filling up our valuable land with waste. Compost amended soil can have beneficial effects by increasing water & nutrient retention in soil, reducing chemical inputs, (toxins, pesticides, etc.) and suppressing plant diseases.

Biodegradable Plastics (BDP) With the biodegradability characteristic BDP is useful mainly in post use stage in waste management utilizing the composter. In agricultural and horticultural use or the civil engineering and construction use BDP products has its advantage not to need to collect the waste products.

  1. Compostable plastic is plastic which undergoes degradation by biological processes during composting to yield carbon dioxide, water, inorganic compounds, and biomass at rate consistent with other compostable material and leaves no visible, distinguishable or toxic residue. (ASTM D6400-99). Compostable biodegradable plastics must be demonstrated to biodegrade and disintegrate in a compost system during the composting process (typically around 12 weeks at temperatures over 50°C). The compost must meet quality criteria such as heavy metal content, ecotoxicity, and no obvious distinguishable residues caused by the breakdown of the polymers. Compostable plastics are a subset of biodegradable plastics. Compostable plastic is a plastic that undergoes biological degradation at a rate consistent with other known compostable materials, such as cellulose, leaving no visually distinguishable or toxic residues.  Certified compostable bioplastics by ASTM are required to degrade within 180 days in a commercial composting facility.  Home composting systems may take more than 180 days to biodegrade.  Based on the thickness, kind of resins, the bioplastics can have varying rates of decomposition.   The corn cutlery, cold cups, drinking straws and the biobags can take up to 180 days in a commercial composting facility.  The potato cutlery will also take 180 days or more to degrade in a commercial facility.  Biodegradation will be faster if the products are broken down to small pieces or ground up.  For all bioplastics apart from the biobags, a commercial compost facility is recommended. Composting programmes are required to effectively and economically dispose PLAbased commodities. NatureWorks (Cargill) company has started buy-back policy for PLA containers and to appropriately dispose off them. This may attract customers.
  1. Hydro-biodegradable and
  2. photo-biodegradabe polymers are broken down in a two-step process – an initial hydrolysis or photo-degradation stage, followed by further biodegradation. Single degradation phase ‘water-soluble’ and ‘photodegradable’ polymers also exist.
  3. Bioerodable – Many polymers that are claimed to be ‘biodegradable’ are in fact ‘bioerodable’ and degrade without the action of micro-organisms – at least initially. This is also known as abiotic disintegration, and may include processes such as dissolution in water, ‘oxidative embrittlement’ (heat ageing) or ‘photolytic embrittlement’ (UV ageing).

 

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